The man surrendered to U.S. military personnel in the Central African Republic, where they are helping African troops hunt for Kony and his fighters, State Department spokeswoman Jen Psaki said. The man said he was an LRA defector and later identified himself as senior commander Dominic Ongwen.
Ongwen is considered by some to be Kony's deputy commander. He is wanted by the International Criminal Court on war crimes and crimes against humanity. Kony's rebellion, which began in Uganda, is accused of some of the world's worst atrocities including mass killings and keeping girls as sex slaves.
"Efforts to establish full and positive identification continue," Psaki told reporters in Washington. "If the individual proves to be Ongwen, his defection would represent a historic blow to the LRA's command structure."
Psaki said the African Union, with U.S. help, was making great strides in eliminating the LRA threat.
Ongwen is being held in the eastern town of Obo in the Central African Republic, Ugandan Army spokesman Paddy Ankunda told The Associated Press.
Kasper Agger, an LRA researcher with Enough Project, an anti-genocide group, described as Ongwen as part of the Lord's Resistance Army's "core command structure," having started with the group in 1990 as a 10-year-old abductee.
Originating in Uganda in the 1980s as a tribal uprising the government, the LRA's rebellion has become one of Africa's longest and most brutal. At the peak of its powers the group razed villages, raped women and amputated limbs. It is especially notorious for recruiting boys to fight and taking girls as sex slaves.
Military pressure forced the LRA out of Uganda in 2005. The rebels scattered across parts of Central Africa. The insurgency and the Ugandan government's response have left at least 100,000 people dead. The U.N. Security Council said in 2011 the conflict had uprooted more than 440,000 people across the region.
Three years ago, President Barack Obama announced he would send 100 U.S. special forces to help the international pursuit of Kony and other Lord's Resistance Army leaders.
Kony remains at large. African forces thought they had Ongwen cornered in August 2012, only to lose him after a shootout with rebels. At the time, it was believed Ongwen sneaked into Congo to avoid capture.
The group is now believed to comprise no more than a few hundred fighters.
Guled reported from Nairobi, Kenya. Court.